Approaches for estimating unsaturated soil hydraulic conductivities at various bulk densities with the extended Mualem-van Genuchten model

The Mualem-van Genuchten model has been widely used for estimating unsaturated soil hydraulic conductivity (K _u ) from measured saturated hydraulic conductivity (K _s ) and fitted water retention curve (WRC) parameters. Soil bulk density (ρ _b ) variations affect the accuracy of K _u estimates. In this study, we extend the Mualem-van Genuchten model to account for the ρ _b effect with ρ _b -related WRC and K _s models. We apply two functions (A and B) that relate the van Genuchten WRC model to ρ _b and two models (1 and 2) that estimate K _s with various ρ _b . By combining the ρ _b -related WRC functions and K _s models, we develop four integrated approaches (i.e., A1, A2, B1, and B2) for estimating K _u at various ρ _b . K _u measurements made on five soils with various textures and ρ _b are used to evaluate the accuracy of the four approaches. The results show that all approaches produce reasonable K _u estimates, with average root mean square errors (RMSEs) less than 0.35 (expressed in dimensionless unit because logarithmic K _u values are used). Approach A2, with an average RMSE of 0.25, agrees better with K _u measurements than does Approach A1 that has an average RMSE of 0.28. This is because Model 2 accounts for the WRC shape effect near saturation. Approaches A1 and A2 give more accurate K _u estimates than do Approaches B1 and B2 which both have average RMSEs of 0.35, because Function A performs better in estimating WRCs than does Function B. The proposed approaches could be incorporated into simulation models for improved prediction of water, solute, and gas transport in soils.